Chopped glass fiber strand is a known useful product for such uses as reinforcement of plastics. It is known to chop glass fiber strand directly as it is being attenuated from a fiber forming bushing. In U.S. Pat. No. 3,869,268, which is incorporated herein by reference, glass filaments are attenuated from a bushing. The filaments are coated with a lubricant binder and/or size as they are attenuated and are gathered by a gathering shoe into one or more unified strands. The unified strands so formed are chopped between a pair of rollers, one of which contains cutting blades and the other of which is used as a backup roller. The resulting chopped strand is conveyed along a pair of vibrating trays under a heater to dry the wet chopped strand. It has been found that with some lubricant binders and/or sizes that, due to heat sinks formed by the reinforcement ribs and vibrator connections under the second vibratory tray, sticking of the chopped strand to the tray often occurs.
In copending U.S. patent application Ser. No 638,481 filed Dec. 8, 1975, assigned to the assignee of the present invention and which is incorporated herein by reference, an insulated vibratory tray is disclosed which comprises a pair of sections having their only connection at their outer lips and being spaced from each other such that an insulating air space between the sections is maintained. The reinforcing ribs are connected to the lower section thus eliminating any heat sinks from the upper conveying section across which the glass strand travels.
With the development of the insulated tray a new problem has arisen. The heaters employed to dry the chopped strand supply enough heat to heat the vibratory tray to a point where no sticking occurs and to dry the chopped glass strand. The heater is operated at a constant, pre-set level is sufficient to both heat the tray and to dry the chopped glass strand.
The system operates sufficiently when a continuous film of glass strand across the tray is maintained. This is, of course, the most desirable way to operate the system. However, it is not always possible to continuously operate the system. For example, break outs of the glass fiber strands will occur for numerous reasons, such as a flooding of the bushing requiring a shut down until the problem is rectified. When the continuous operation is interrupted, no chopped glass strand will be traveling along the conveyor. When this occurs, no heat is required to dry chopped glass strand. This heat is, however, being supplied by the heaters, since they have been pre-set to a specific output level. Much of this unused heat will be absorbed by the conveyor thus increasing its temperature above that which is desired. When resumption of flow of the glass strand occurs, the extra heat causes the binder and/or size to become overcured thus resulting in unacceptable products.
It is desirable, therefore, to be able to control the temperature of the conveyor such that its temperature is not permitted to rise above a certain level even when the flow of glass strand is interrupted.